1. Field of the Invention
The present invention relates to an image forming apparatus (e.g., a copying machine, a printer, and a facsimile apparatus) that is operable according to an electrophotographic method.
2. Description of the Related Art
A tandem-type image forming apparatus is a conventionally known image forming apparatus that includes a plurality of image forming units sequentially disposed along a rotational path of an intermediate transfer member to form a full-color image using the electrophotographic method.
According to an intermediate transfer mechanism of the tandem-type image forming apparatus, the intermediate transfer member constantly contacts an electrophotographic sensitive member (i.e., a photosensitive member) at a primary transfer portion while the intermediate transfer member is moving along the rotational path thereof. Therefore, contact portions of the intermediate transfer member and the photosensitive member are gradually worn down due to friction and contact pressure. This will cause deteriorations in surface characteristics of the intermediate transfer member and the photosensitive member. In a case where the image forming apparatus performs a black monochrome image forming operation, there is a photosensitive member that is not used. If the non-used photosensitive member is forcibly caused to contact the intermediate transfer member, the photosensitive member will have an unnecessarily shortened life. In particular, if the conveyance speed of the photosensitive member is different from that of the intermediate transfer member, the abrasion amount of a surface layer of the photosensitive member will excessively increase and, as a result, the lifetime of the photosensitive member will be shortened.
On the other hand, it is conventionally known that an image forming apparatus can include a mechanism operable in a black monochrome mode to bring an intermediate transfer member out of contact with photosensitive members of other color image forming units. Further, Japanese Patent Application Laid-Open No. 2010-66452 discusses an image forming apparatus, especially a full-color copying machine for office use, in which only a photosensitive drum of a black image forming unit (i.e., a most-frequently used image forming unit) has a larger diameter.
Further, to solve a dropout problem, it is also conventionally known that an elastic intermediate transfer belt made of an elastic layer is employable as the intermediate transfer member. In general, the dropout possibly occurs when a larger pressure is applied to toner particles in a transfer operation because a part of the toner particles cannot be transferred to the intermediate transfer member and remains on the photosensitive member.
However, when an image forming unit is equipped with a larger-diameter photosensitive member, image defectiveness tends to occur due to abnormal discharge or deterioration in transfer property.